1) Field of the Invention
The present invention relates to an optical integrated circuit formed by interconnecting a plurality of optical components, and more specifically, relates to an optical circuit and an optical integrated circuit module. In the optical integrated circuit module, optical hybrid integrated devices, in which a planar lightwave circuit having optical waveguides formed on a PLC platform, and a semiconductor device having optical active components, such as semiconductor laser diodes and semiconductor photodiodes formed on a semiconductor substrate are coupled with each other.
2) Description of the Related Art
With the spread and progress of optical communication networks, functions of optical components for use in optical transmission systems have been sophisticated. The optical components include optical active components for emitting or receiving optical signals, optical passive components for splitting/coupling or demultiplexing/multiplexing the optical signals, optical fibers for use in transmission lines of the optical signals, or the like, and an improvement in performance or a reduction in cost is increasingly required for respective optical components. Among these, with regard to the optical active components, devices based on semiconductor materials, such as semiconductor lasers and semiconductor photodiodes are the main devices, and technical development thereof has been advanced. The optical active component based on the semiconductor material has features of allowing optical amplification function, high-speed operation, and compact integration. Meanwhile, with regard to the optical passive components, planar lightwave circuits (PLC; Planar Lightwave Circuit, which will be referred to PLC hereinbelow) having optical waveguides based on silica-based materials are commercially produced. PLC has advantageous features of allowing optical waveguides to realize with low loss and without polarization dependency.
While improvement in performance of respective elements has been independently made for both the optical active element and the optical passive element until now, requirement for high performance optical components having both advantages has been increased because of sophisticated needs resulting from development of the optical transmission systems. Therefore, developments of optical hybrid integrated devices in which semiconductor active elements (optical active component) such as semiconductor laser diodes or the like, and PLC are combined with each other have been made.
In a conventional art disclosed in, for example, Patent Document 1, a semiconductor laser diode is hybrid-mounted on the PLC platform, and thus achieving a laser that oscillates in an external resonator mode which is formed between the semiconductor laser diode and a UV grating on the PLC. In this conventional art, there is only one waveguide for introducing a light outputted from the semiconductor laser diode into the UV grating. Therefore, one end facet of the emitting waveguide (a semiconductor waveguide on the Si terrace for mounting the laser diode) of the semiconductor laser diode and one end facet of the optical waveguide on the PLC are coupled with each other.
Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2001-267684
Meanwhile, in a conventional art disclosed in following Document 2, an optical wavelength selector is achieved by hybrid-integrating an arrayed waveguide grating (AWG) on the PLC and semiconductor optical amplifiers (SOAs). Here, SOAs are used as gate switches, wherein input waveguides and output waveguides of SOAs are in contact with different end facets of the semiconductor substrate, and in contact with the PLC platform at respective end facets to optically couple with the optical waveguides on the PLC platform.
Document 2: I. Ogawa, F. Ebisawa, N. Yoshimoto, K. Takiguchi, F. Hanawa, T. Hashimoto, A. Sugita, M. Yanagisawa, Y. Inoue, Y. Yamada, Y. Tohmori, S. Mino, T. Ito, K. Magari, Y. Kawaguchi, A. Himeno, and K. Kato, “Lossless hybrid integrated 8-ch optical wavelength selector module using PLC platform and PLC-PLC direct attachment techniques” Proc. OFC' 98, 1998, paper PD4-1
Moreover, in following Document 3, there is disclosed a technology in which waveguides on different PLC platforms (a first PLC platform and a second PLC platform) are optically coupled with each other. In this conventional art, the waveguides on one PLC has a turnaround portion. However, since it is difficult to achieve a high refractive index difference in PLC, there is no choice other than setting a radius of curvature of the bent waveguide in the turnaround portion to a quite large value.
Document 3: Japanese Unexamined Patent Publication (Kokai) No. H10-227936
However, when the semiconductor element, such as a SOA having the input waveguide and the output waveguide, and the optical waveguides on right and left PLCs existing on both sides of the semiconductor element are coupled with each other as the conventional art disclosed in aforementioned Document 2, following fixing is required. Namely, one end facet of the semiconductor substrate with the end of the input waveguides is fixed to the end facet of one PLC platform, and the other end facet of the semiconductor substrate with the end of the output waveguides is also fixed to the end facet of the other PLC platform. As a result, the input waveguides of the semiconductor elements are coupled with the optical waveguides of one PLC, and the output waveguides thereof are coupled with the optical waveguides of the other PLC. As described above, the number of surfaces (contact surfaces) for fixing the semiconductor substrate which has the input waveguides and the output waveguides and on which the semiconductor elements is formed, and the PLC platform is increased. In this case, it is necessary to obtain excellent couplings between the waveguide on the semiconductor substrate and the optical waveguides on the right and left PLC platforms at respective contact surfaces. For this reason, in the conventional art described in aforementioned Document 2, optical alignment works between the waveguides must be performed at two contact surfaces. One of two contact surfaces is a contact surface between one end facet of the semiconductor substrate and the end facet of one PLC platform, and another is a contact surface between the other end facet of the semiconductor substrate and the end facet of the other PLC platform, respectively. Hence, since the man-hour for alignment increases in this conventional art, the optical alignment works will be troublesome and take time, and a possibility that alignment mistakes may occur will also be increased. As a result, there has been a problem of difficulty in obtaining the excellent coupling efficiency.
Meanwhile, it is conceivable to insert the semiconductor elements into an area (cutout portion) where a part of the optical waveguides on the one PLC platform is cut off, and then arrange it. However, even in this case, there have been problems that a dimensional accuracy to a length of the cutout portion or the semiconductor element would be severe, or the optical alignment works would be complicated and difficult, in order to make the coupling efficiency between the input side and the output side waveguides of the semiconductor element, and the optical waveguides on the PLC platform excellent.
Further, when those hybrid-integrated circuit of the semiconductor element and PLC platforms is modularized with fiber pigtails or fiber arrays, the optical alignment works between the PLC platforms and the fibers need to be performed at two points of the end facets of the PLC platforms and the man-hour for alignment increases by that much. As a result, the optical alignment works will be troublesome and time consuming, and the possibility that optical alignment mistakes may occur will also be increased, thus causing the problem of difficulty in obtaining the excellent coupling efficiency.